Geophysical detection of outbursts

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Geophysical techniques provide the opportunity for remote assessment of the state of a rock mass and to remotely monitor changes in its behaviour. Geophysical techniques are widely used for these purposes in resource exploration and extraction. Because coal outbursts can be associated with geological structures and with zones of anomalous states of saturation and stress, geophysical techniques can be used to detect such situations. There may also be precursors to coal outbursts which can be detected using geophysical techniques.

If geophysical techniques are used for outburst management, there are caveats to be considered. Geophysical images only provide general images of the state of the rock mass. Precise mapping of outburst prone structures is not possible. Outbursts can also occur in situations where anticipated geophysical precursors do not occur. Geophysical techniques should therefore be applied as a complement to other approaches to predicting outbursts and mitigating outburst risks.

Detection of faults and sheared zones
Seismic reflection surveying
Geological faults with throws greater than about 3 m are usually detectable using seismic reflection surveying from the ground surface. Smaller faults are not detectable on account of the relatively large wavelengths of the seismic waves. Furthermore, not all faults are associated with outbursts. The presence of sheared zones containing crushed coal (mylonite) is usually required. Seismic reflection surveys are unlikely to detect the presence of mylonite. Geological knowledge of the likely style of faulting and whether sheared zones exist is therefore useful.

Another approach to the detection of faults using seismic methods is to the use in-seam seismic technique. This technique was trialed in Australia through the 1980’s. Seismic waves are launched into the coal seam and form a guided wave which travels out through the coal seam. Faults and other seam disruptions will generate reflections which indicate the location of the disruption. This technique is now rarely used in Australia.

Logging in-seam boreholes
One approach to the detection of faults with smaller throws is to utilise geophysical techniques from within in-seam boreholes. Development of prototype directional radar tools was funded by ACARP (C8016). For these tools the radar wavelengths allow the mapping of joints and fractures but the penetration is at best, a few metres. One problem with radar systems is that if the water in the borehole is saline, strong borehole reverberations are generated and little energy is launched into the coal rock mass.

In-seam boreholes also provide the opportunity for the deployment of geophysical logging tools that map the state of the coal in the borehole wall. Mylonite zones are believed to contain much more moisture than intact coal and tools which detect changes in moisture can therefore be employed. A dielectric logging tool with such an objective in mind was developed through ACARP (C6026). Conventional resistivity tools might also have role.

Mylonite zones can also be expected to give rise to borehole caving which can be detected by borehole caliper tools (ACARP C3071). In a similar way, the monitoring of drilling parameters such as torque and thrust is likely to record drilling anomalies at the intersection of weak coal in mylonite zones. This potential was investigated in ACARP projects C3073 and C7023.

A common issue for the development of all underground in-seam borehole logging systems concerns their deployment. Tools need to be intrinsically safe or have operational safeguards that allow certification for their use. Deployment in sub-horizontal holes is also an issue and means of tool delivery to the end of boreholes is required. In principle the logging can be integrated into the drilling.

One way in-seam logging and measurement of drill parameters may become routinely applied in the Australian coal industry is via the drilling of surface to in-seam holes. These holes do not carry the same requirements for intrinsic safety. Tools developed for use in the petroleum industry can also be considered. ACARP projects C12024 and C14034 have successfully tested a number of geophysical tools directed towards investigating down-hole geological conditions in surface to in-seam boreholes.

Detection of igneous intrusions
Discontinuities in coal seams at the margins of dykes and other igneous intrusions can also be the sites of outbursts. Igneous intrusions can be investigated by geophysical surveys.

Airborne and ground surface magnetic surveys are frequently conducted to locate dykes that might disrupt underground mining. The main limitations in these surveys are that the results mainly reflect the locations of the intrusions at the ground surface. Their locations at coal seam level require extrapolation. Many dykes are vertical but they can also be offset. Local geological knowledge is therefore required to help appraise results.

Sills at coal seam depths of a few hundred metres are very difficult to detect from the air and ground surface by magnetic surveys. One possibility for their detection is through use of electromagnetic survey methods developed for the exploration for metallic ore deposits that are electrically conductive. Cindered coal at the margins of igneous intrusions, can contain graphite, an extremely conductive mineral. If the zones containing graphite are metres wide, then it is possible that they can be detected from the ground surface. (King, 1987)

Detection of zones poorly drained of gas
In Australia, outburst risk is currently mitigated mainly through gas drainage strategies. Development of appropriate strategies and the monitoring of their effectiveness are therefore important requirements. Geophysical methods can have a role in monitoring the effectiveness of gas drainage by virtue of the fact that water needs to be drained from coal before gas will flow. Undrained coal retains its moisture and electrically, wet coal is more conductive than dry coal. These changes in conductivity can be remotely detected using geophysical methods.

Radio imaging method (RIM)
RIM is a technique based on exploiting changes in the electrical conductivity within coal and between the coal and the roof and floor strata. As with the in-seam seismic method, coal seams act as a waveguide and electromagnetic waves launched into a coal seam are guided by it. Changes in the conductivity of the coal, or the surrounding rocks change the propagation characteristics of the wave. Increases in wave attenuation occur in zones of wet coal and also when coal seams are faulted or intruded. Tomographic imaging similar to that used in medical imaging can be used to map the changes in conductivity.

To interpret RIM results, independent geological insights into the range of possible anomalies are required. In the context of outburst assessment, RIM surveys can indicate the locations of faults with throws greater than about half seam thickness and dykes of thickness greater than about 1 m. Independently of these constraints, RIM surveys can detect zones of width greater than a few metres which contain in-situ water and are therefore outburst prone. Such zones can include ‘hard to drain zones’ such as sheared zones where permeability pathways are blocked.

In Australia, RIM was thoroughly investigated through NERDDC project C1209. Many mines also conducted RIM surveys through the 1990’s though not many surveys are currently undertaken.

Monitoring for outburst precursors
Investigation and monitoring of seismic precursors to rock bursts has been an important activity in many deep and highly stressed metalliferous mines, world-wide. The growth of fractures leading to a rock burst creates seismic emissions. The location and properties of these emissions allows assessment of the physical state of the rock mass and the likelihood of spontaneous failure. Coal-bursts (as distinct to outbursts) are a form of such failure.

Associations between seismic activity and outbursts appear to be much more dependent on the situations at individual sites. Styles (1995) reports the ability to record from the ground surface narrow-band seismic events believed to be associated with the resonances associated with the desorption of gas into developing fractures. These events are also of low frequency and audible underground. In this work, mining was stopped until activity ceased.

Other outbursts do not to appear to have been associated with such precursory seismic activity. However, Hatherly et al (1995) report seismic events at Tahmoor Colliery of a similar nature which were related to an increase in gas make in the mine ventilation but no outburst. Some success in relating microseismic emissions to increased cross-measure gas make was also reported by ACARP project C6025.

These two examples of microseismic monitoring involved monitoring with the sensors placed at or near the ground surface. There have been other examples of microseismic monitoring involving equipment and sensors underground. With sensors within metres of the face there is the potential to record much higher seismic frequencies. Acoustic emissions directly associated with fracture growth may be detected. However with this type of measurement, the issues of the certification of the instruments arise and there are also operational considerations when deploying instruments at the active face.

From eastern Europe and Russia, there have been many studies into seismic monitoring and outbursts. Some results are presented in Lama (1995).

Another approach into monitoring rock failure processes involves seismo-electric effects. Through a number of mechanisms that are not particularly well understood, rock failure can also lead to the generation of electromagnetic emissions. Vozoff through ACARP project C9005 investigated electromagnetic emissions generated during goaf formation at Moonee Colliery. Outburst may also cause precursory electromagnetic emissions. In principle, electromagnetic measurements are easier to make because direct coupling between the sensors and the rock are not required. However the issues of certification and operational expediency in underground operations still arise.

State of stress
Just as with rock bursts, associations between outbursts and high stress gradients at the working face can be investigated by microseismic monitoring. With appropriate instrumentation, it is possible to locate seismic events in real-time and make an assessment as to the state of stress in the rock mass (current ACARP project C15024). It is also possible to use the seismic noises created by shearers to develop tomographic images of seismic velocity and seismic attenuation in the near vicinity of the face. The velocity and attenuation can be related to the stress in the rock mass. ACARP project C15023 is currently investigating this approach.